Here's the test and results:
I used a feeler gauge (0.010") which appears to be brass and checked its thickness using a Starret No. 3732XFL-1 mic (see pic; it's zero'd). The mic's info pamphlet says it's resolution is .00005" and it's accuracy is plus or minus .0001".
After acetone cleaning the feeler gauge with microfiber cloth, drying it and again checking for zero on the mic, ten readings of the gauge thickness were .00985" for six of the readings and .00990" for four of the readings, n=10, for a mean of .00987, median .009085, and SD .0000258".
Then I used a Sharpie fine point black permanent marker (the ink comes off brass with acetone; same marker I use to number my cases) and wrote "3" (once) on one part of the feeler gauge (see pic) and re-checked that the mic was zero'd. After allowing the ink to dry, repeated readings (n=9) directly on the "3" using the mic were .00985" for five readings and .00990" for four readings; a mean of .00987", median of .00985" and SD of .0000264".
Essentially, using this mic, the thickness of the feeler gauge with and without a single "3" mark were the same. However, I because I was getting mic values that very slightly differed (.00900 vs .00985) when repeating the same measurement, I think I was at or near the limit of the mic's accuracy.
Therefore I performed the same test again but this time I wrote "3" at the same location on the feeler gauge five times each superimposed on each other and allowing the "3" to completely dry each time to my eye (about 1 min); see pic where the "3" appears darker than when applied once. It's possible that some of the ink came off (dissolved onto the pen) as I superimposed each of the "3's" but looking at it each time, I don't think so.
Measurements of thickness of the gauge without the five "3s" was .00990" for all of ten readings, and .01010" with the five "3s" for all of five readings. The difference was .00020", and divided by 5 the result was .00004". These data suggest each application of the "3" was about .00004" thick which to me seems negligible for purposes of preparing rounds for precision shooting.
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